Magnetic memory device and methods for making same

ABSTRACT

In one embodiment, a memory device includes a plurality of magnetic data cells and a magnetic reference cell extending uninterrupted along more than one of the plurality of data cells.

THE FIELD OF THE INVENTION

The present invention generally relates to nonvolatile memory devices,and more particularly to memory devices that use magnetic memory cells.

BACKGROUND OF THE INVENTION

One type of nonvolatile memory known in the art relies on magneticmemory cells. Known as magnetic random access memory (MRAM) devices,these devices include an array of magnetic memory cells. The magneticmemory cells may be of different types. For example, a magnetic tunneljunction (MTJ) memory cell or a giant magnetoresistive (GMR) memorycell.

The typical magnetic memory cell includes a layer of magnetic film inwhich the magnetization is alterable and a layer of magnetic film inwhich the magnetization is fixed or “pinned” in a particular direction.The magnetic filn having alterable magnetization may be referred to as asense layer or data storage layer and the magnetic film that is fixedmay be referred to as a reference layer or pinned layer.

Conductive traces (commonly referred to as word lines and bit lines, orcollectively as write lines) are routed across the array of memorycells. Word lines extend along rows of the memory cells, and bit linesextend along columns of the memory cells. Located at each intersectionof a word line and a bit line, each memory cell stores the bit ofinformation as an orientation of a magnetization. Typically, theorientation of magnetization in the data storage layer aligns along anaxis of the data storage layer that is commonly referred to as its easyaxis. External magnetic fields are applied to flip the orientation ofmagnetization in the data storage layer along its easy axis to either amatching (i.e., parallel) or opposing (i.e, anti-parallel) orientationwith respect to the orientation of magnetization in the reference layer,depending on the desired logic state.

The orientation of magnetization of each memory cell will assume one oftwo stable orientations at any given time. These two stableorientations, parallel and anti-parallel, represent logical values of“1” and “0”. The orientation of magnetization of a selected memory cellmay be changed by supplying current to a word line and a bit linecrossing the selected memory cell. The currents create magnetic fieldsthat, when combined, can switch the orientation of magnetization of theselected memory cell from parallel to anti-parallel or vice versa.

FIGS. 1 a through 1 c illustrate the storage of a bit of data in asingle memory cell 20. In FIG. 1 a, the memory cell 20 includes anactive magnetic data film 22 and a pinned magnetic film 24 which areseparated by a dielectric region 26. The orientation of magnetization inthe active magnetic data film 22 is not fixed and can assume two stableorientations as shown by arrow M₁. On the other hand, the pinnedmagnetic film 24 has a fixed orientation of magnetization shown by arrowM₂. The active magnetic data film 22 rotates its orientation ofmagnetization in response to electrical currents applied to the writelines (130,132, not shown) during a write operation to the memory cell20. The first logic state of the data bit stored in memory cell 20 isindicated when M₁ and M₂ have matching (i.e, parallel) orientations asillustrated in FIG. 1 b. For instance, when M₁ and M₂ have matchingorientations, a logic “1” state is stored in the memory cell 20.Conversely, a second logic state is indicated when M₁ and M₂haveopposite (i.e, anti-parallel) orientations as illustrated in FIG. 1 c.When the orientations of M₁ and M₂ are opposite each other, a logic “0”state is stored in the memory cell 20. In FIGS. 1 b and 1 c thedielectric region 26 has been omitted. Although FIGS. 1 a through 1 cillustrate the active magnetic data film 22 positioned above the pinnedmagnetic film 24, their positions may be reversed.

The logic state of the data bit stored in the memory cell 20 can bedetermined by measuring its resistance. The resistance of the memorycell 20 is reflected by a magnitude of a sense current 23 (referring toFIG. 1 a) that flows in response to read voltages applied to the writelines 30, 32.

In FIG. 2, the memory cell 20 is positioned between the write lines 30,32. The active and pinned magnetic films 22,24 are not shown in FIG. 2.The orientation of magnetization of the active magnetic data film 22 isrotated in response to a current I_(x) that generates a magnetic fieldH_(y) and a current I_(y) that generates a magnetic field H_(x). Themagnetic fields H_(x) and H_(y) act in combination to rotate theorientation of magnetization of the memory cell 20.

As illustrated in the above Figures, the layers of magnetic material aretypically formed as geometrically patterned films such as squaresellipses, or rectangles. One disadvantage of patterned magnetic layerstorage structures is that patterned magnetic layers generate amagnetostatic field that tends to demagnetize the layer. Thisdemagnetizating field tends to reorient the magnetization of the thinfilm so as to minimize the energy of the patterned element, the endresult being a non-uniform or multi-domain magnetization state.Magnetostatic fields from patterned layers also interact with magneticmaterial in proximity to the edges of the patterned film, potentiallydisrupting the magnetization state in the proximate magnetic material.For example, referring to FIG. 1 a, the magnetization M₂ of pinnedmagnetic film 24 creates a demagnetization field in a direction opposingM_(2.) This field interacts with data film 22 and biases the magnetichysteresis loop of the data film such that the hysteresis loop may nolonger be symmetric about zero field. In a memory application thisoffset can be very damaging. If the offset is greater than thecoercivity of the data film, then there is loss of data after removal ofthe writing field. An offset field lower than the coercivity is alsodetrimental in that it introduces assymetry into the writing process.Any variation in this offset field adversely affects the writing marginwhen attempting to write a single data film within an array of memoryelements.

When reading the magnetic memory elements, non-uniform magnetization ormultiple domains tend to create noise or areas of varying resistanceacross the memory cell that makes determination of the state of thememory cell difficult or impossible. In addition, variation in thedomain states can produce fluctuations in the switching field that canrender the memory cell writing process unpredictable. From the above, itcan be seen that maintaining a uniform magnetization direction in themagnetic layers is important. In the case of the fixed magnetization ofthe reference layer, it is thus desirable to pin the magnetization in amanner that minimizes the presence of magnetostatic fields that mayinteract in a deleterious manner with the data film.

SUMMARY OF THE INVENTION

One embodiment of a memory device comprises a plurality of magnetic datacells and a magnetic reference cell extending uninterrupted along morethan one of the plurality of data cells.

One method for creating a memory device comprises depositing a referencelayer, depositing a separation layer over the reference layer, anddepositing a sense layer over the separation layer. The sense layer ispatterned to form a plurality of data cells, and the separation layerand reference layer are patterned to form a plurality of elongatedreference cells. Each of the plurality of elongated reference cellsextends past more than one of the plurality of data cells.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a through 1 c are profile and side illustrations of a simplifiedmagnetic memory cell illustrating an orientation of magnetization ofactive and reference magnetic films.

FIG. 2 is a profile block diagram of a prior art magnetic memory cell,its write lines, and magnetic fields generated by currents flowingthrough the write lines.

FIG. 3A is a perspective illustration of a memory device according toone embodiment of the invention.

FIG. 3B is a perspective illustration of a memory device according toanother embodiment of the invention.

FIG. 4 is a perspective illustration of a memory device according toanother embodiment of the invention.

FIG. 5 is a perspective illustration of a memory device according toanother embodiment of the invention.

FIG. 6 is a perspective illustration of a memory device according toanother embodiment of the invention.

FIG. 7 is a perspective illustration of a memory device according toanother embodiment of the invention.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which likenumerals are used for like and corresponding parts of the drawings.

The several embodiments of magnetic memory devices described hereininclude a reference cell which has a known stable magnetic orientationin a magnetic random access memory (MRAM) stack. The orientation ofmagnetization of the reference cell is maintained by introducingmagnetic anisotropy into the reference cell. Magnetic anisotropy refersgenerally to the exhibition of preferred directions of magnetization ina magnetic material. The introduction of magnetic anisotropy ensuresthat the magnetization of the reference layer remains pinned in thedesired orientation when subjected to magnetic fields normallyencountered in an MRAM device. In addition, magnetic anisotropysignificantly reduces the tendency for the magnetization of thereference cell to break up into multiple domains. For example,variations in the reference cell composition or shape contribute to themagnetic anisotropy observed. Alternatively, unidirectional anisotropycan be introduced to the reference cell by coupling a ferromagneticlayer to an antiferromagnetic layer.

Embodiments of the invention minimize or eliminate demagnetizationfields associated with the reference layer in the vicinity of the datacells. As will be shown in the subsequent detailed descriptions, this isaccomplished in one embodiment by employing an elongated reference cellthat reduces or eliminates the effects of demagnetization fieldsassociated with the peripheral edges of the reference cell.

One embodiment of a magnetic memory device 100 according to theinvention is shown in FIG. 3A. Memory device 100 includes referencecells 124 which extend uninterrupted along a plurality of data cells122, with a separation or barrier layer 126 positioned between thereference cells 124 and the data cells 122. The reference cells 124 maylie substantially along a write conductor 130. A second write conductor132 lies substantially perpendicular to write conductor 130. Writeconductors 130,132 are shown in FIG. 3A to be positioned such that theyare in contact with data cells 122 and reference cells 124. However, inalternate embodiments write conductors may be spaced by some distancefrom data cells 122 and reference cells 124. Additional conductors (notshown) may optionally be provided to separate the “read” and “write”functions of the conductors.

The magnetic orientation of the reference cell 124 is pinned by shapeanisotropy. Shape anisotropy is created by increasing one dimension(e.g., length) of a ferromagnetic layer with respect to anotherdimension (e.g., width) of the layer. The introduction of shapeanisotropy helps to ensure predictability with respect to theorientation of the magnetic vectors within the reference cell.

Shape anisotropy describes the influence of geometry on the directionaldependence of the ability to magnetize an otherwise magneticallyisotropic sample in an applied magnetic field. Generally, magnetizationof a film along its shortest dimension (i.e., across its width) is moredifficult because the demagnetizing field is greatest in that direction.For a rectangular magnetic element of thickness T and width W (wherelength>>W), the shape anisotropy H_(shape) of the element isapproximated as:H _(shape)=4πM _(s) T/Wwhere M_(s) is the saturation magnetization of the element.

Magnetization is constrained to be aligned with the long dimension ofthe element by shape anisotropy, which can be on the order of at leastseveral hundred Oersteds for materials typically envisioned for use inmagnetic memory cells. For example, for a NiFeCo film (4πM_(s) equalsapproximately 12000 Gauss) the shape anisotropy H_(shape) is about 600Oersteds for a 10 mm thick film having a 0.2 μm width. The shapeanisotropy of an Fe film (4πM_(s) equals approximately 21000 Gauss)would be even greater. This shape anisotropy is significant enough toforce the magnetization to always lie along the length of the patternedreference layer.

In the reference cells 124, the orientation of magnetization M liesalong the length of the reference cell 124. In the data cells 122, theeasy axis of the cells lies parallel to the orientation of magnetizationM in the reference cells 124. The length-to-width ratio of eachreference cell 124 is substantially larger than the length-to-widthratio of the data cells 122. In one embodiment according to theinvention, the reference cells 124 have a length-to-width ratio of atleast 4:1, the width of the elongated reference cells 124 is in therange of approximately 0.05-5.0 μm, and the data cells 122 havedimensions in the range of approximately 0.05-1.0 μm. The separationlayer 126 may be either a conductive material or a non-conductivematerial, depending upon the type of memory cell to be constructed.

In another embodiment according to the invention, the magneticorientation of the reference cells 124 is defined to lie along the longdimension of the elongated reference cell 124 (refer to FIG. 3A) bymagnetocrystalline anisotropy alone, or by a combination of shape andmagnetocrystalline anisotropies. Magnetocrystalline anisotropy refers tothe influence of material composition and crystallographic orientationon the directional dependence of the ability to magnetize a sample in amagnetic field. The films commonly selected for use in magneticnonvolatile memory applications exhibit uniaxial magnetocrystallineanisotropy. The material is easier to magnetize along a particular axis.This axis is typically referred to as the “easy axis” of the film. Whilethe magnetocrystalline anisotropy of NiFe is only about 5 Oe, “hard”magnetic alloys, for example CoPt, can have magnetocrystallineanisotropies of hundreds, or even thousands, of Oersteds. Theorientation of the easy axis can be defined by deposition in a magneticfield, by post-deposition annealing in a magnetic field, or by controlof the crystallographic orientation.

In yet another embodiment the magnetic orientation of reference cells124 is defined to lie along the long dimension of the elongatedreference cell 124 (refer to FIG. 3B) by exchange coupling theferromagnetic reference layer to an antiferromagnetic layer 128.Exchange coupling the reference cell 124 to an antiferromagnetic layer128 introduces a uniaxial anisotropy into the elongated reference cell124, giving the magnetization of the reference layer a unique preferredorientation. The anisotropy direction can be specified by deposition ina magnetic field or by post-deposition magnetic field annealing.Examples of antiferromagnetic materials for this application are IrMn,FeMn, PtMn, CrPtMn NiMn, NiO and Fe2O3. Uniaxial anisotropies of severalhundred Oersteds can be introduced in this manner.

The elongated reference layer in FIGS. 3A and 3B need not be comprisedof only a single ferromagnetic film. Another embodiment of a magneticmemory device 200 according to the invention is shown in FIG. 4. Writeconductors 130, 132 are not shown for purpose of clarity, but would besituated similarly to those shown in FIG. 3. In the embodiment of FIG.4, memory device 200 uses two or more ferromagnetic layers to form areference cell 224, with each pair of adjacent ferromagnetic layersseparated by a non-magnetic layer. In FIG. 4, reference cell 224 isshown to include two ferromagnetic layers 224 a, 224 b separated by anon-magnetic spacer layer 228. In alternate embodiments according to theinvention, additional ferromagnetic and spacer layers may be provided.

Similar to the earlier-described embodiments, the reference cell 224 ispatterned in an elongated manner, and the easy axes of the ferromagneticlayers are oriented along the long dimension. Definition of the easyaxis direction can be done by deposition in an applied magnetic field orby post-deposition magnetic field annealing, for example.

The non-magnetic spacer 228 is preferably chosen from a group ofmaterials that are known to mediate exchange coupling between twoferromagnetic layers. Examples of suitable materials include Cu, Cr, Ru,Re, and Os. The exchange coupling is known to oscillate betweenferromagnetic and antiferromagnetic as a function of the thickness ofspacer 228. A preferred spacer thickness produces antiferromagneticcoupling between the ferromagnetic layers 224 a, 224 b. The preferredthickness is dependent on the particular spacer material, but isgenerally less than approximately 5 nm, and can be as little as 0.4 nm.The thickness of each of the ferromagnetic layers is typically less than10 nm. Examples of suitable ferromagnetic materials are NiFe, Co, Fe,CoFe, NiFeCo, CrO2 and Fe3O4.

In the embodiment of FIG. 4, each elongated reference cell 224 extendsuninterrupted along a plurality of data cells 222, with separation orbarrier layer 226 positioned between reference cell 224 and data cells222. The two ferromagnetic layers 224 a, 224 b of the reference cell 224are coupled such that the orientations of magnetization M′ and M″ of thereference cells 224 a and 224 b are parallel to the long dimension ofthe reference cell 224. Because the orientations of magnetization M′, M″may be ambiguous, in one embodiment according to the invention one offerromagnetic layers 224 a, 224 b is thicker than the other topositively determine the orientations of magnetization M′ and M″ alongthe reference cell 224.

Yet another embodiment of a magnetic memory device 300 according to theinvention is shown in FIG. 5. As in FIG. 4, write conductors 130, 132are not shown for purpose of clarity, but would be situated similarly tothose shown in FIGS. 3A and 3B. Memory device 300 is constructedsubstantially the same as memory device 200 shown in FIG. 4, and likecomponents are similarly numbered.

The embodiment according to the invention shown in FIG. 5 positivelydetermines the directions of magnetizations M′ and M″ across referencecell 224 by adding an antiferromagnetic layer 230 immediately adjacentreference cell 224. Specifically, ambiguity in the preferredmagnetization orientations M′ and M″ of the reference layer 224 isremoved by coupling the outermost ferromagnetic layer 224 b in themultilayer reference stack to antiferromagnetic layer 230. Couplingferromagnetic layer 224 b to antiferromagnetic layer 230 defines themagnetization orientation M″ of ferromagnetic film layer 224 b. Since inthe preferred case adjacent ferromagnetic layers are oriented opposingone another (i.e, anti-parallel), the magnetization orientation M′ offerromagnetic layer 224 a in the multilayer stack is also defined.

In the foregoing embodiments the easy axes of the data cells are alongthe long dimension of the elongated reference cell, and themagnetization of the elongated reference cell is constrained to liealong its long dimension. This configuration eliminates demagnetizationfields from the reference layer, since there are no gradients inmagnetization present within the elongated reference layer. Hence, thedata cells have no magnetostatic fields from the reference layer.

An alternative embodiment of a memory device 400 that contains multipleferromagnetic layers as the reference cell is presented in FIG. 6.Similar to the earlier described embodiments, the reference cell 324 ispatterned in an elongated manner. However, in contradistinction to theearlier described embodiments, the preferred magnetization orientationof the ferromagnetic layers 324 a, 324 b which form reference cell 324in this case is perpendicular to the long dimension of the elongatedreference cell 324 (i.e., across the width of the reference cell 324,rather than along the length of reference cell 324).

A magnetization orientation perpendicular to the long dimension of theelongated reference cell can be realized by having the easy axis of theferromagnetic layers in this direction. Definition of the easy axisdirection can be done by deposition in an applied magnetic field or bypost-deposition magnetic field annealing, for example. After patterniingthe elongated reference cell, magnetostatic interactions between thepairs of ferromagnetic layers will tend to further stabilizeantiferromagnetic alignment between ferromagnetic pairs. Similar to theembodiment of FIG. 4, the non-magnetic spacer layer is selected from amaterials set known to enhance the antiferromagnetic orientation.

Reference layers 324 a, 324 b of the memory device 400 shown in FIG. 6rely on magnetocrystalline anisotropy to maintain the orientation ofmagnetization in the reference cell 324. In the embodiment of FIG. 6,each elongated reference cell 324 extends uninterrupted along aplurality of data cells 322, with separation or barrier layer 326positioned between reference cell 324 and data cells 322. The twoferromagnetic layers 324 a, 324 b of the reference cell 324 areseparated by non-magnetic spacer layer 328 and coupled such that theorientations of magnetization M′ and M″ of the reference cells 324 a and324 b are perpendicular to the long dimension of the reference cell 324.Because the orientations of magnetization M′, M″ may be ambiguous, inone embodiment, according to the invention, one of ferromagnetic layers324 a, 324 b is thicker than the other to positively determine thedirection of the orientations of magnetization M′ and M″ across thereference cell 324. In the data cells 322, the easy axis of the cells322 lies parallel to the orientation of magnetization M′ in thereference cell 324.

As the width of the reference cell 324 decreases, the opposingmagnetization orientations of ferromagnetic layers 324 a, 324 b isfurther stabilized by strong magnetostatic coupling between the layers.In one embodiment according to the invention, the reference cell 324 hasa width of less than approximately 5 μm, and is preferably less thanapproximately 0.3 μm in width

A final embodiment of a magnetic memory device 500 according to theinvention is shown in FIG. 7. As in FIG. 6, write conductors 130, 132are not shown for purpose of clarity, but would be situated similarly tothose shown in FIG. 3. Memory device 500 is constructed substantiallythe same as memory device 400 shown in FIG. 6, and like components aresimilarly numbered.

The embodiment according to the invention shown in FIG. 7 positivelydetermines the direction of orientation of magnetizations M′ and M″across reference cell 324 by adding an antiferromagnetic layer 330immediately adjacent reference cell 324. Specifically, ambiguity in thepreferred magnetization orientations M′ and M″ of the reference layer324 is removed by coupling the outermost ferromagnetic layer 324 b inthe multilayer reference stack to antiferromagnetic layer 330. Couplingferromagnetic layer 324 b to antiferromagnetic layer 330 defines themagnetization orientation M″ of ferromagnetic film layer 324 b. Sinceadjacent ferromagnetic layers are always oriented with their magneticorientations opposing one another, the magnetization orientation M′ offerromagnetic layer 324 a in the multilayer stack is also defined.

The memory devices 100, 200, 300, 400, 500 described herein maygenerally be created using semiconductor processing techniques known inthe art. In one method for creating the memory devices, a grating ofconductors (the write conductors) is formed using any suitable techniqueknown in the art. A stack of material for creating the memory cell(reference cell 124, 224, 324, separation or barrier layer 126, 226, 326and data cell 122, 222, 322) is deposited in an unpatterned conditionover the conductors. Next, the upper layer is patterned to form aplurality of individual data cells 122, 222, 322. The separation layer126, 226, 326 and reference cells 124, 224, 324 are then patterned toform a plurality of elongated reference cells 124, 224, 324 where eachreference cell 124, 224, 324 extends past more than one of the pluralityof data cells 122, 222, 322.

The terms “patterning” and “pattern” as used herein refer to the removalof material by any means, including but not limited to ion etching,reactive ion etching, or wet chemical etching. If the reference cells124, 224, 324 rely on shape anisotropy to maintain a stable orientationof magnetization, the layer of material used to form reference cells124, 224, 324 may be formed by depositing only a single layer offerromagnetic material. If the reference cells 124, 224, 324 useantiferromagnetic coupling to maintain a stable orientation ofmagnetization, the reference cells 124, 224, 324 may be formed bydepositing a first layer of magnetic material, depositing a layer ofnon-magnetic material over the first layer of magnetic material, andthen depositing a second layer of magnetic material over the layer ofnon-magnetic material (for example, corresponding to ferromagnetic layer224 b, separation layer 228, and ferromagnetic layer 224 a,respectively). As is known in the art, the ferromagnetic materials maybe deposited in the presence of a magnetic field to help establish apreferred magnetic orientation.

In some instances, it may be desired that the data cells 122, 222, 322be located at the bottom of the stack of materials, rather than at thetop as described in the method above. In this situation, the memorydevices 100, 200, 300, 400, 500 may be formed by depositing a layer offerromagnetic material, and then patterning that layer to form aplurality of data cells 122, 222, 322. A separation layer 126, 226, 326and layer of material used to form reference cells 124, 224, 324 maythen be deposited over the patterned data cells 122, 222, 322. The layerof material used to form reference cells 124, 224, 324 may either bedeposited in a patterned manner, or may be deposited and then patternedto form the elongated reference cells 124, 224, 324. As noted above, ifthe reference 124, 224, 324 rely on antiferromagnetic coupling tomaintain a stable orientation of magnetization, the layers of materialsused to form reference cells 124, 224, 324 may be formed by depositing afirst layer of magnetic material, depositing a layer of non-magneticmaterial over the first layer of magnetic material, and then depositinga second layer of magnetic material over the layer of non-magneticmaterial (corresponding to ferromagnetic layer 224 b, separation layer228, and ferromagnetic layer 224 a, respectively).

1-26. (canceled)
 27. A method for creating a memory device comprising:depositing a reference layer; depositing a separation layer over thereference layer; depositing a sense layer over the separation layer;patterning the sense layer to form a plurality of magnetic data cells;and patterning the separation layer and reference layer to form anelongated magnetic reference cells, wherein the elongated magneticreference cells extends uninterrupted along more than one of theplurality of magnetic data cells.
 28. The method of claim 27, whereindepositing the reference layer comprises: depositing a first layer ofmagnetic material; depositing a layer of non-magnetic material over thefirst layer of magnetic material, and depositing a second layer ofmagnetic material over the layer of non-magnetic material.
 29. A methodfor creating a memory device comprising: depositing a sense layer;patterning the sense layer to form a plurality of magnetic data cells;depositing a separation layer over the plurality of data cells;depositing a reference layer over the separation layer; and patterningthe reference layer to form an elongated magnetic reference cell whereinthe elongated magnetic reference cell extends uninterrupted along morethan one of the plurality of magnetic data cells.
 30. The method ofclaim 29, wherein depositing the reference layer comprises: depositing afirst layer of magnetic material; depositing a layer of non-magneticmaterial over the first layer of magnetic material; and depositing asecond layer of magnetic material over the layer of non-magneticmaterial. 31-34. (canceled)
 35. The method of claim 27, whereinpatterning the separation layer and reference layer to form an elongatedmagnetic reference cell comprises providing the elongated magneticreference cell with a length-to-width ratio of at least 4:1.
 36. Themethod of claim 27, further comprising aligning a magnetic orientationof the elongated magnetic reference cell with an easy axis of each ofthe more than one of the plurality of magnetic data cells along whichthe reference cell extends.
 37. The method of claim 27, whereindepositing a separation layer over the reference layer comprisesdepositing a layer of non-conductive material over the reference layer.38. The method of claim 28, wherein depositing a first layer of magneticmaterial comprises depositing a layer of magnetic material having afirst thickness, and wherein depositing a second layer of magneticmaterial comprises depositing a layer of magnetic material having asecond thickness different than the first thickness.
 39. The method ofclaim 28, wherein depositing a layer of non-magnetic material comprisesdepositing a material selected from the group consisting of Cr, Cu, Ru,Re, and Os.
 40. The method of claim 28, further comprising providing thefirst layer of magnetic material with a first orientation ofmagnetization, and providing the second layer of magnetic material witha second orientation of magnetization, wherein the first orientation ofmagnetization and the second orientation of magnetization are oppositeto one another.
 41. The method of claim 27, further comprising pinningan orientation of magnetization of the elongated reference cell by shapeanisotropy.
 42. The method of claim 27, further comprising pinning anorientation of magnetization of the elongated reference cell bymagnetocrystalline anisotropy.
 43. The method of claim 29, whereinpatterning the reference layer to form an elongated magnetic referencecell comprises providing the elongated magnetic reference cell with alength-to-width ratio of at least 4:1.
 44. The method of claim 29,further comprising aligning a magnetic orientation of the elongatedmagnetic reference cell with an easy axis of each of the more than oneof the plurality of magnetic data cells along which the reference cellextends.
 45. The method of claim 29, wherein depositing a separationlayer over the plurality of data cells comprises depositing a layer ofnon-conductive material over the plurality of data cells.
 46. The methodof claim 30, wherein depositing a first layer of magnetic materialcomprises depositing a layer of magnetic material having a firstthickness, and wherein depositing a second layer of magnetic materialcomprises depositing a layer of magnetic material having a secondthickness different than the first thickness.
 47. The method of claim30, wherein depositing a layer of non-magnetic material comprisesdepositing a material selected from the group consisting of Cr, Cu, Ru,Re, and Os.
 48. The method of claim 30, further comprising providing thefirst layer of magnetic material with a first orientation ofmagnetization, and providing the second layer of magnetic material witha second orientation of magnetization, wherein the first orientation ofmagnetization and the second orientation of magnetization are oppositeto one another.
 49. The method of claim 29, further comprising pinningan orientation of magnetization of the elongated reference cell by shapeanisotropy.
 50. The method of claim 29, further comprising pinning anorientation of magnetization of the elongated reference cell bymagnetocrystalline anisotropy.